Three batteries in series to get to 12V.Two such strings in parallel for longer runtime.Usually no need to separate different batteries for different motors.Also, what is the C rating of the batteries?A LiPo can typically drive its mAh rating in amps well - this is known as "1 C."Many modern batteries can drive more (but for a shorter duration.) For example, a 20 C, 2000 mAh battery could drive up to 40A of current without overheating/safety concerns.